The Grand Chamber of the European Court of Human Rights (ECHR) has agreed to hear a legal challenge to the use of bulk data collection surveillance powers by U.K. intelligence agencies.

Last September, a lower chamber of the ECHR ruled that U.K. surveillance practices violated human rights law but did not find bulk collection itself to be in violation of the convention.

The civil and digital groups and charities behind the challenge, which include Liberty, Privacy International and Amnesty International, are hoping for a definitive judgement against bulk collection from Europe’s highest human rights court.

The legal challenge dates back around five years, and stems from the 2013 disclosures of government surveillance programs revealed by NSA whistleblower Edward Snowden .

The ECHR’s lower court heard an amalgam of complaints from three cases. And in a landmark judgement last fall it found the U.K.’s bulk interception regime had violated Article 8 of the European Convention on Human Rights (a right to respect for private and family life/communications); and Article 10 (the right to freedom of expression and information).

The court found there was insufficient oversight of the internet infrastructure and communications selected for interception and searching; and also insufficient safeguards for journalistic material.

The court also ruled against the government’s regime for obtaining data from communications service providers, finding it violated both articles.

But the judges declined to find the state surveillance regime unlawful on the grounds that it constituted “general and indiscriminate” retention of data.

This is important because the legal framework around surveillance in the U.K. had already been superseded — with the Investigatory Powers Act, which was passed in 2016 — enshrining a number of bulk powers in law, alongside what the government bills as an adequate oversight framework. (Though it has since been forced by domestic courts to rework certain aspects of the legislation judged to be disproportionate.)

The groups behind the human rights challenge argue the lower court’s judgment “did not go far enough with regard to the unlawfulness of bulk interception powers and the fundamental shortcomings in inter-state intelligence sharing based on communications intercepts.”

Hence, now pushing for an overarching judgement from judges in the Grand Chamber which — if it goes their way — could force the U.K. to radically rethink its approach to intelligence capabilities and put a check on the creeping encroachment of state surveillance.

Commenting in a statement, Caroline Wilson Palow, general counsel at Privacy International, said: “The UK Government continues to intercept enormous volumes of internet traffic flowing across its borders. And it continues to have access to similarly vast troves of information intercepted by the US Government. We call on the Court to reject these mass surveillance practices and find that they are fundamentally incompatible with the rights to privacy and freedom of expression enshrined in the European Convention on Human Rights.”

“The surveillance regime that the UK Government has built seriously undermines our freedom. Spying on vast numbers of people without suspicion of wrongdoing violates everyone’s rights to privacy and free expression, and can never be lawful,” added Megan Goulding, lawyer for Liberty, in another statement. “We welcome the opportunity from the Court to prove that indiscriminate state snooping is incompatible with our rights. We need a rights-respecting and targeted surveillance system — not one where everyone is treated as a suspect as they go about their everyday lives.”

Also commenting in a statement, Lucy Claridge, director of strategic litigation at Amnesty International, said: “Industrial scale mass surveillance makes it incredibly difficult for organisations such as Amnesty International to carry out their vital human rights work. It’s critical that they are able to seek and receive information of public interest from their confidential sources, free from government intrusion.”

There’s little prospect of an imminent check on the U.K.’s current bulk-based surveillance modus operandi via this legal route, with what could be a wait of several years before the Grand Chamber even hears the case.

Add to that, at that unknown future time it’s still anyone’s guess whether the U.K. — which is in the process of trying to determine how it will exit the European Union — will still be a party to the European Convention on Human Rights or not.

While the ECHR is attached to the Council of Europe, rather than the EU itself, some elements of the Conservative Party have been pushing to pull the U.K. out of the convention too. Which throws a potential future spanner in the works of this rights-based challenge.